Regeneration of chelated polyvalent metal solutions by controlled potential electrolysis

ABSTRACT

Electrochemical regeneration in an electrochemical cell of spent scrubbing solutions containing polvalent metal chelates is accomplished without substantial degradation of the chelate by controlling the anolyte pH and maintaining the anode potential of the cell above the oxidation potential of the polyvalent metal chelate but below the oxidation potential of the chelate portion of the polyvalent metal chelate.

BACKGROUND OF THE INVENTION

(1) Field of the invention

The present invention relates generally to the regeneration of a spentscrubbing solution containing a polyvalent metal chelate subsequent toits use in removal of hydrogen sulfide from a sour, gaseous stream.

(2) Description of the prior art

The use of an aqueous chelated polyvalent metal catalyst solution forremoving hydrogen sulfide from a sour, gas stream is well known in theart. The chelated polyvalent metal aqueous solution upon contact withthe hydrogen sulfide-containing gas stream effects oxidation of thehydrogen sulfide to elemental sulfur and at the same time the polyvalentmetal chelate is reduced to a lower valence state. Most frequently thepolyvalent metal utilized is iron. Regeneration of the chelatedpolyvalent metal solution by the oxidation of the ferrous chelate to theferric chelate is most generally accomplished by contacting the solutionwith an oxygen-containing gas. U.S. Pat. No. 4,622,212 to McManus et alprovides a representative listing of United States patents illustratingprocesses for removal of hydrogen sulfide from a sour gaseous streamusing a polyvalent metal chelate. In McManus et al, excessivedegradation during regeneration of the chelating agent is presented byincorporation of a stabilizing agent such a an alkaline thiosulfate inthe polyvalent metal chelate scrubbing solution. This patent is herebyincorporated by reference as providing a particularly through discussionof the early prior art showing regeneration of spent polyvalent metalchelate with an oxygen containing gas. A more recent patent in this areais U.S. Pat. No. 4,696,802 to Bedell.

In U.S. Pat. No. 4,455,287 to Primack et al, a method of stabilizing achelated polyvalent metal, utilized in a scrubbing solution for removinghydrogen sulfide from a gas stream, is disclosed as the use of a broadspectrum biocide. This prevents degradation of the chelated polyvalentmetal caused by contamination of the aqueous composition withmicroorganisms such as bacteria.

In U.S. Pat. No. 4,532,118 to Tajiri et al, the degeneration of achelated polyvalent metal utilized in a scrubbing solution for removalof hydrogen sulfide from a gas stream is referred to as causing theformation of ferrous sulfide which when mixed with the free sulfurproduct causes a darkening in color of the sulfur, thereby impairing thecommercial value thereof. The means suggested by Tajiri et al ofreducing degradation of the polyvalent metal chelate is to adjust themole ratio of ferric ion to total iron ions in the scrubbing solution inthe range of 0.6 to 0.9.

Regeneration of spent scrubbing solutions utilizing an electrochemicalcell is also known in the art. In U.S. Pat. No. 4,126,529 to DeBerry,the spent ferric chelate containing scrubbing solution utilized toremove oxides of nitrogen and sulfur from flue gases is regenerated bypassing the solution through the cathode compartment of anelectrochemical cell. The regeneration process involves the removal ofthe sulfate ions from the scrubbing solution through the ion transfermembrane and the reduction of the non-reactive ferric chelate to thereactive ferrous chelate. In U.S. Pat. No. 4,076,793 to Nikolai a methodis disclosed for removing sulfur dioxide from a gas stream utilizing anaqueous slurry of manganous hydroxide as an absorbent. Manganous sulfateis regenerated to manganous hydroxide by aqueous phase electrochemicalprecipitation from the spent scrubbing solution. In U.S. Pat. No.4,041,129 to Foster et al, a process is disclosed for removal of acidicgases from hydrocarbon streams in which regeneration of the scrubbingsolution is disclosed as utilizing an electrolytic cell. In this processan aqueous sodium hydroxide solution is utilized to scrub the acidicgases. The rich, scrubbing solution is thereafter reacted with aqueoussulfuric acid to liberate the acidic gases such as hydrogen sulfide andthe resulting aqueous sodium sulfate solution is convertedelectrolytically to sodium hydroxide at the cathode of anelectrochemical cell.

Regeneration of spent hydrogen sulfide scrubbing solutions utilizing afuel cell is disclosed in U.S. Pat. Nos. 4,320,180 to Nozaki and4,436,713 and 4,436,711 both to Olson. The Olson patents indicate thatdegradation of the polyvalent metal chelate may be caused by or enhancedby regeneration of the spent scrubbing solution with oxygen or anoxygen-containing gas and that regeneration utilizing a fuel cell avoidsthis problem.

Regeneration of a hydrogen sulfide spent gas scrubbing solutioncontaining a polyvalent metal chelate by the use of an electrolytic cellis disclosed in U.S. Pat. No. 4,643,886 to Chang et al and in the OlsonU.S. Pat. Nos. 4,436,712 and '714 and 4,443,423 and '424. Each of thesepatents teaches reduced degradation of the polyvalent metal chelateduring regeneration of the chelate in an electrolytic cell as comparedto regeneration using oxygen or an oxygen-containing gas.

In none of the prior art references in which a polyvalent metal chelateis regenerated utilizing an electrolytic cell or a fuel cell is theelectrical potential imposed upon the electrolytic cell controlled suchthat the potential of the cell is maintained at a value lower than theoxidation potential of the polyvalent metal chelate. The applicant istherefore the first to discover that all degradation of a polyvalentmetal chelate can be avoided during regeneration of a polyvalent metalchelate from a lower valence state to a higher valence state in theanode compartment of an electrolytic cell simply by maintaining theanode potential below the oxidation potential of the chelate portion ofthe polyvalent metal chelate being regenerated.

SUMMARY OF THE INVENTION

In accordance with the present invention, it has now been found that bycontrolling the anode potential of an electrochemical cell utilized toregenerate a spent, gas scrubbing solution containing at least onepolyvalent metal chelate, regeneration can be accomplished generallywithout substantial degradation of the chelate portion of the polyvalentmetal chelate, and preferably without degradation. In the process of theinvention for converting a lower valence state polyvalent metal chelateto a higher valence state polyvalent metal chelate, the anode potentialof the electrochemical cell is controlled to a value which is below theoxidation potential of the chelate portion of the polyvalent metalchelate and above the oxidation potential of the polyvalent metalchelate. The regeneration reaction is particularly suitable for practicein an electrochemical cell of the invention in which the rate ofreactivity is commercially practical in spite of the low cell potentialrequired. In the electrochemical cell of the invention, an increasedregeneration rate is attained by the use of a high surface area anode.Also disclosed is a method for preventing degradation of a polyvalentmetal chelate present in a spent gas scrubbing solution by the use of anelectrolytic cell to effect regeneration and a process for removinghydrogen sulfide from a sour gaseous stream utilizing an electrolyticcell for regeneration of the spent gas stream scrubbing solution.Preferably, the process is continuous.

DETAILED DESCRIPTION OF THE INVENTION

It has been generally recognized that the most effective polyvalentmetal chelates for removal of hydrogen sulfide from a process stream arebased upon amino polycarboxylic acid chelating agents, for instance,ethylenediamine tetraacetic acid (EDTA) and its homologs. It is thistype of chelating agent that degrades rapidly in a continuous processfor removal of hydrogen sulfide when the spent chelated polyvalent metalscrubbing solution is regenerated using oxygen or an oxygen-containinggas. In the regeneration step, the polyvalent metal is converted fromthe lower valence state to the higher valence state in which it isactive as an oxidizing agent for hydrogen sulfide to convert this gas toelemental sulfur. In the contact zone, the polyvalent metal chelate isreduced from the higher valence state of the metal to the lower valencestate of the metal so that the coordination number of the metal isreduced.

In accordance with the invention, a process is disclosed for removal ofhydrogen sulfide from a sour, fluid stream and conversion to elementalsulfur. A polyvalent metal chelate is utilized in an aqueous, alkaline,scrubbing solution, preferably in a continuous process, to convert thehydrogen sulfide to sulfur. Regeneration is accomplished in anelectrolytic cell wherein the anodic potential of the cell is controlledso as to prevent degradation of the polyvalent metal chelate. Inaddition, a process is disclosed for preventing degradation of apolyvalent metal chelate present in a spent, scrubbing solution used toremove hydrogen sulfide from a sour, fluid stream wherein the polyvalentmetal chelate is regenerated in an electrolytic cell and subsequentlyrecycled to a contact zone for use in oxidizing hydrogen sulfide tosulfur. An electrolytic cell is disclosed which is particularly suitablefor the regeneration of a polyvalent metal chelate present in a spent,gas stream, scrubbing solution for removal of hydrogen sulfide from asour, fluid stream wherein a porous anode is utilized and the cell isoperated with a controlled anodic potential in order to avoiddegradation of the polyvalent metal chelate.

In the process of the invention, generally a hydrogen sulfidecontaining, fluid stream, i.e., a gas stream is contacted in a contactzone with an aqueous, alkaline solution containing a polyvalent metalchelate wherein the metal is in the oxidized, i.e., the higher valence,state. The particular type of fluid stream treated is not critical aswill be evident to those skilled in the art. Streams particularly suitedto removal of H₂ S by the practice of the invention arenaturally-occurring hydrocarbon gas streams, synthesis gases, refineryprocess gases, and fuel gases produced by gasification procedures, e.g.,gases produced by the gasification of coal, petroleum, shale, tar sands,etc. Particularly preferred are coal gasification streams, natural gasstreams and refinery feedstocks composed of gaseous hydrocarbon streams,and other gaseous hydrocarbon streams. The term "hydrocarbon gasstream(s)", as employed herein, is intended to include streamscontaining significant quantities of hydrocarbon (both paraffinic andaromatic), it being recognized that such streams contain significant"impurities" not technically defined as a hydrocarbon. Streamscontaining principally a single hydrocarbon, e.g., ethane, are eminentlysuited to the practice of the invention. Streams derived from thegasification and/or partial oxidation of gaseous or liquid hydrocarbonsmay be treated by the invention. The H₂ S content of the type of streamscontemplated will vary extensively, but, in general, will range fromabout 0.1 percent to about 10 percent by volume. The amount of H₂ Spresent is not generally a limiting factor in the practice of theinvention.

Temperatures employed in the contact zone wherein hydrogen sulfide isabsorbed utilizing an aqueous, alkaline solution are not generallycritical, except that the reaction is generally carried out at atemperature below the melting point of sulfur. The preferred operatingtemperature range is from about 10° centigrade to about 70° centigrade.The most preferred temperature range is from about 25° to about 50°centigrade. At higher temperatures, the rate of polyvalent metal chelatedegradation increases to unacceptable levels. At lower temperatures,reaction kinetics slow down while hydrogen sulfide absorption increases,which can cause a chemical imbalance to occur. In addition, at lowertemperatures soluble components of the aqueous alkaline solution may beprecipitated from solution. Contact times in the contact zone generallyrange from about 1 second to about 270 seconds or longer, with contacttimes of about 2 seconds to about 120 seconds being preferred.

A feature of the process of the invention is the regeneration of apolyvalent metal chelate in an electrolytic cell under controlledanolyte pH as well as controlled anodic potential conditions.Regeneration is accomplished, subsequent to separation of sulfur fromthe rich, aqueous, alkaline, scrubbing solution, by passing the spentscrubbing solution to the anode compartment of an electrochemical cell.In this cell, the operating range for anolyte pH is generally from abut6.5 to about 7.5. The preferred range is about 6.9 to about 7.1. Ingeneral, an anolyte pH of about 7 is preferred in order to avoiddegradation of the polyvalent metal chelate during regeneration. Ingeneral, the desired anodic potential must be determined for eachspecific polyvalent metal chelate. For instance, for ethylene diaminetetra-acetic acid, the anodic potential must be below +0.4.

The pH operating range in the contact zone of the process generally canbe from about 6 to about 10, the preferred range being from about 7 toabout 9, and the most preferred range being from about 8 to about 9. Thecontact zone of the process is operated generally at the highestpossible pH within the above contact zone pH range in order to operateat a high efficiency of hydrogen sulfide absorption. Since hydrogensulfide is an acid gas, the aqueous alkaline solution upon absorption ofthe hydrogen sulfide in the contact zone is lowered in pH. The optimumpH depends upon the particular polyvalent metal chelating agent,particularly the polyvalent metal utilized therein. Thus the ability ofthe polyvalent metal chelating agent to protect the metal fromprecipitation as an insoluble sulfide or hydroxide at high pH valueswill determine how high in pH the aqueous alkaline solution in thecontact zone can be used. For instance, at pH values below about 6, theefficiency of absorption of hydrogen sulfide with an iron chelate in thecontact zone is so low as to be impractical. While at pH values greaterthan about 10, the precipitation of insoluble iron hydroxide occursresulting in decomposition of the chelated iron.

Another feature of the process of the invention and the method ofoperating an electrolytic cell to regenerate a polyvalent metal chelatelies in the operation of the electrolytic cell at a controlled anodicpotential. Specifically, an anodic potential above the oxidationpotential of the lower valence state polyvalent metal chelate but belowthe oxidation potential of the chelate portion of the lower valencestate polyvalent metal chelate. In operating the hydrogen sulfideabatement process of the invention in this manner, pure, soluble sulfuris produced since degradation of the polyvalent metal chelate isavoided. In the prior art, electrochemical regeneration processes inwhich an anodic potential is used which is above the oxidation potentialof the polyvalent metal chelate, some of the polyvalent metal chelate isdestroyed and unchelated iron is released, which in turn reacts with aportion of the sulfide which is present. The resulting dark colored ironsulfide contaminates the sulfur produced in the process and destroys itsvalue.

In the process of the invention for the abatement of hydrogen sulfidefrom a sour fluid stream, the rich, scrubbing solution is led to aseparation zone in which the elemental sulfur in said solution isrecovered by any of the conventional separation processes known forrecovery of elemental sulfur from aqueous solutions. For example, thesulfur can be recovered by flocculation and settling, centrifugation,filtration, flotation, and the like. The method of sulfur recovery isnot critical to the process of the invention. It is desirable to recoveras much as possible of the aqueous, alkaline, scrubbing solution tominimize physical losses of the polyvalent metal chelate.

After sulfur separation, the aqueous, alkaline, scrubbing solution ispassed as an electrolyte to the anode compartment of an electrolyticcell containing a permselective cell membrane and a porous anode.Preferably, the anode is a porous graphite anode. A caustic solution isrecirculated through the cathode chamber of the electrolytic cell duringthe regeneration operation. In the anolyte compartment of the cell, thepolyvalent metal chelate is oxidized from the reduced or lower valencestate to the higher valence state of the polyvalent metal and thereafterthe electrolyte is returned as a lean, scrubbing solution to the contactzone of the process. In order to provide optimum pH conditions in thecontact zone of the process, additional hydroxide can be added to theanolyte prior to use in the contact zone of the process. The aqueous,alkaline solution utilized in the contact zone upon absorption ofhydrogen sulfide is lowered in pH and thereby becomes more suitable forregeneration under optimum pH conditions in the electrolytic cell of theinvention.

Any oxidizing polyvalent metal chelating agent can be used in theprocess of the invention but those in which the polyvalent metal isiron, copper, and manganese are most preferred, particularly iron. Otheruseful metals which can provide the polyvalent metal of the polyvalentmetal chelating agent are generally those that are capable of undergoinga reduction- oxidation reaction. Generally, those metals are suitablewhich are capable of being reduced to a lower valence state by reactionwith hydrosulfide or sulfide ions and which can be regenerated byoxidation with an oxygen containing gas to a higher valence state.Specific examples of preferred useful metals include, besides the mostpreferred metals listed above, nickel, chromium, cobalt, tin, vanadium,platinum, palladium, and molybdenum. The metals are normally supplied inthe form of a salt, oxide, hydroxide, etc.

The useful polyvalent metal chelates, which can be used singly or incombination, are coordination complexes in which the polyvalent metalsform chelates generally by reaction with an amino carboxylic acid, anamino polycarboxylic acid, a polyamino carboxylic acid, or a polyaminopolycarboxylic acid. Preferred coordination complexes are thosepolyvalent metals which form chelates with an acid having the formula:##STR1## wherein two to four of the X groups are lower alkyl carboxylicgroups, zero to two of the X groups are selected from the groupconsisting of lower alkyl groups, hydroxyalkyl groups, and ##STR2## andwherein R is a divalent organic group. Representative divalent organicgroups are ethylene, propylene, isopropylene or alternativelycyclohexane or benzene groups where the two hydrogen atoms replaced bynitrogen are in the one or two position, and mixtures thereof.

The polyvalent metal chelates useful in the process of the invention arereadily formed in an aqueous medium by reaction of at least one salt,oxide, or hydroxide of the polyvalent metal and, preferably, an aminocarboxylic acid or an amino polycarboxylic acid which can also bepresent in the form of an alkali metal or ammonium salt thereof.Exemplary amino carboxylic acids include (1) amino acetic acids derivedfrom ammonia or 2-hydroxy aklyl amines, such as glycine, diglycine(imino diacetic acid), NTA (nitrilo triacetic acid), 2-hydroxy alkylglycine; di-hydroxyalkyl glycine, and hydroxyethyl or hydroxypropyldiglycine; (2) amino acetic acids derived from ethylene diamine,diethylene triamine, 1,2-propylene diamine, and 1,3-propylene diamine,such as EDTA (ethylene diamine tetraacetic acid), HEDTA (2-hydroxyethylethylene diamine tetraacetic acid), DEPTA (diethylene triaminepentaacetic acid); and (3) amino acetic acid derivatives of cyclic1,2-diamines, such as 1,2-diamino cyclohexane N,N-tetraacetic acid and1,2-phenylenediamine-N,N-tetraacetic acid. The iron chelates of NTA and2-hydroxyethyl ethylene diamine triacetic acid are preferred.

The polyvalent metal chelate is used in the contact zone of the processgenerally in an effective amount suitable for oxidizing substantiallyall the hydrogen sulfide removed from the hydrogen sulfide containinggas scrubbed in the contact zone of the process by the aqueous alkalinescrubbing solution. Preferably, an amount is used of about 2 moles toabout 10 moles of polyvalent metal chelate per mole of hydrogen sulfideabsorbed by the aqueous alkaline solution, although an amount up to thesolubility limit of the polyvalent metal chelate in the aqueous alkalinesolution can be used. Most preferably, about 2 moles to about 5 moles ofpolyvalent metal chelate per mole of absorbed hydrogen sulfide is used.

The buffering agents which are useful as optional components of theaqueous, alkaline, lean, scrubbing solution of the invention are ingeneral those which are capable of stabilizing said aqueous, alkaline,scrubbing solution to a pH in the desired operating pH range ofgenerally about 6 to about 10. The buffering agents should be watersoluble at the concentrations in which they are effective. Examples ofsuitable buffering agents optionally used in the process of theinvention are the alkali metal salts of carbonates, bicarbonates, orborates. Examples of useful specific buffering agents within theseclasses of buffering agents are sodium carbonate-bicarbonate or sodiumborate. Where the hydrogen sulfide containing feed gas also containscarbon dioxide at a volume percent of greater than about 5%, thecarbonate-bicarbonate buffers are the preferred buffers for use in theprocess of the invention. These may be produced insitu by the additionof a base such as sodium hydroxide in the preparation of the aqueous,alkaline, scrubbing solution. Where the hydrogen sulfide containing feedgas contains carbon dioxide only in a minor amount, (less than about5%), then the borate buffers, for example, borax or sodium borate (Na₂B₄ O₇) are useful.

Hydrogen sulfide absorbents can be employed in the contact zone toincrease the absorptivity of the aqueous, alkaline, scrubbing solution.Any of the known absorbents conventionally used which do not affect theactivity of the polyvalent metal chelate can b used. The hydrogensulfide solvent can be either a physical solvent or a regenerablechemical solvent but a physical solvent is preferred. The vapor pressureof the hydrogen sulfide solvent should be low enough so that it is notstripped from the solution in substantial amounts in the contact zone ofthe process. The hydrogen sulfide solvent can be either an organic or aninorganic solvent or a solvent which, in combination with the aqueous,alkaline solution, increases the solubility of the combined solutionwith respect to hydrogen sulfide. Examples of suitable hydrogen sulfidesolvents are as follows: tripotassium phosphate, tributyl phosphate,tetrahydrothiophene dioxide, dimethyldithiodipropionate,N-methyl-2-pyrrolidone, N-methylpyrrolidine, N-formylmorpholine,N-formyldimethylmorpholine, N,N-dimethylformamide, propylene carbonate,dialkyl ethers of polyethylene glycols, and dimethyl or diethyl glycinesalts. The particular hydrogen sulfide absorbent chosen is a matter ofchoice given the qualification that the hydrogen sulfide solvent mustnot effect the activity of the polyvalent metal chelate and the hydrogensulfide solvent must exhibit sufficient solubility for hydrogen sulfide.

The contact zone of the process in which a hydrogen sulfide containinggas stream is contacted with a lean, scrubbing solution containing apolyvalent metal chelate can be operated at ambient conditions oftemperature and pressure. Temperatures from about 5° to about 65° C. andpressures from about subatmospheric to 100 atmospheres or greater can beused.

In the regeneration process of the invention, the electrolytic cell isoperated at an anodic potential which is less than the oxidationpotential of the chelate portion of the specific polyvalent metalchelate used and above the oxidation potential of the polyvalent metalchelate. Preferably the regeneration process is conducted using apolyvalent metal chelate derived from an amino carboxylic acid at ananodic potential below +0.4 volts versus a saturated calomel electrode.The oxidation potential of a polyvalent metal chelate solution isdependent upon the concentration of the oxidized and reduced species ofpolyvalent metal. For instance, the oxidation potentials become morepositive with greater concentrations of the oxidized species present.The rate of oxidation is reduced at a given anode potential as the ratioof oxidized to reduced species of polyvalent metal increases.

The following examples illustrate the various aspects of the inventionbut are not intended to limit its scope. Where not otherwise specifiedthroughout this specification and claims, temperatures are given indegrees centigrade and parts, percentages, and proportions are byweight.

EXAMPLE 1

In order to illustrate the regeneration of a polyvalent metal chelatewithout decomposition in an electrolytic cell, an electrolytic cellcontaining a permselective membrane (Nafion® 324) was utilized togetherwith platinum screen electrodes in an electrolytic cell which wasconnected to a Princeton Applied Research Model 371 potentiostat whichwas coupled to a Princeton Applied Research Model 175 universalprogrammer and an X-Y recorder. The cell was submerged in a constanttemperature water bath held at a temperature of 50° C. A saturatedcalomel reference electrode was used to monitor the anodicelectrochemical potential by way of a saturated potassium chloride saltbridge. The catholyte was 100 milliliters of saturated sodium sulfate.The anolyte was 100 milliliters of a solution containing 0.13 grams offerric sulfate and 0.745 grams of the trisodium salt of N-(hydroxyethyl)ethylene-diaminetriacetic acid in demineralized water.

In order to determine the desired anodic potential for operation of thecell, cyclic volametry studies were conducted between anodic potentialsof -0.3 and +1.2 volts versus a saturated calomel reference electrodewhile current was monitored as a function of potential. A reaction wasdetected at +0.4 volts with a second reaction detected at 0.96 volts, asindicated by the change in slope of the curve at that voltage. Thissecond reaction is interpreted as oxygen evolution. The first reactionis the result of decarboxylation of the trisodium salt ofN-(hydroxyethyl) ethylene-diaminetriacetic acid. Since the oxidationpotential achieved utilizing air oxidation (of a lower valencepolyvalent metal chelate to a higher valence state polyvalent metalchelate) is higher than the decarboxylation potential of the trisodiumsalt of N-(hydroxyethyl)ethylenediaminetriacetic acid, air oxidationwill decompose a polyvalent metal chelate based upon the trisodium saltof N-(hydroxyethyl) ethylene-diaminetriacetic acid. The oxidation of aferrous chelate based upon the trisodium salt ofN-(hydroxyethyl)ethylene- diaminetriacetic acid occurs at a much loweroxidation potential than the degradation reaction of the trisodium saltof N-(hydroxyethyl) ethylene-diaminetriacetic acid, as determined by thecyclic voltametry study.

These results show that a solution of a ferrous chelate based upon thetrisodium salt of N-(hydroxyethyl) ethylene-diaminetriacetic acid can beoxidized electrochemically in an electrochemical cell in which the anodeis operated at a potential above the oxidation potential of thepolyvalent metal chelate and below the oxidation potential of thechelate portion of the polyvalent metal chelate. If operated withinthese limits, the chelating agent is not degraded and a spent gasscrubbing solution containing a lower valence state polyvalent metalchelate can be regenerated to the higher valence state polyvalent metalchelate for subsequent use as a lean, scrubbing solution in a contactzone in which hydrogen sulfide is abated from a fluid stream.

EXAMPLE 2

Cyclic voltametry studies were conducted in an electrolytic cell havinga nickel cathode, a cation exchange membrane and various anodes asfollows: a platinum screen, a ruthenium oxide coated titanium anode, anIrO₂ coated titanium anode, a graphite sheet, and a porous graphiteanode. The rate of oxidation of a ferrous chelate based upon thetrisodium salt of N-(hydroxyethyl) ethylenediaminetriacetic acidutilizing these anodes was measured. The rates of oxidation, asindicated by current density, in amps per square inch, at +0.5 volts,versus a standard calomel electrode, show that the porous graphite anodeis far superior in providing a high rate of oxidation as compared to theother anodes. In addition, the other anodes are more expensive. Theporous, high surface area, graphite anode is relatively inexpensive andreadily available as well as stable and inert under the conditions ofthe electrolytic cell. The current density in amps per square inch at0.5 volts for the various anodes used in this electrolytic cell are asfollows:

platinum screen 0.079

ruthenium oxide on titanium anode 0.031

IrO₂ on a titianium sheet anode 0.032

graphite sheet 0.034

porous graphite anode 0.32

EXAMPLE 3

An electrochemical cell was used to demonstrate that electrochemicalregeneration of a ferric chelate based upon the trisodium salt ofN-(hydroxyethyl) ethylenediaminetriacetic acid could be achieved withoutchelate degradation. The electrochemical cell consisted of two parallel1O.4 square inch plate electrodes, a ruthenium oxide coated titaniumanode, and a stainless steel cathode. A cation exchange membrane soldunder the trade designation NAFION 324 was used to separate thecatholyte and anolyte compartments. The electrolyte gaps between theelectrodes and membrane wire one-eighth inch in width. The cell wasfitted with a saturated calomel reference electrode to monitor the anodepotential. An ECO Instruments Model 550 potentiostat was used to supplypower to the cell. A model 721 ECO Instruments Integrator monitored thecoulombs passed. One liter of one molar sodium sulfate was used as thecatholyte. The anolyte used was one liter of 0.38 weight percent iron ina 5.9 weight percent solution of the trisodium salt ofN-(hyroxyethyl)ethylene-diaminetriacetic acid. The ferrous iron chelatein the amount of 56 grams per liter was transferred to a mixer and 20milliliters of a one molar solution of NaHS were added to react with 50percent of the ferrous iron. After mixing, the solution was filtered toremove the sulfur and the anolyte was transferred to the electrolyzerreservoir. The electrolytes were pumped through the cell until about3900 coulombs were passed to reoxidize the reduced (ferrous) iron. Thisprocess was repeated twelve times using the same chelate solution. Theanolyte was sampled after every four cycles and analyzed for theconcentration of the trisodium salt ofN-(hydroxyethyl)ethylene-diaminetriacetic acid Samples of the sulfurproduced were also submitted for analysis. The results obtained indicatethat no significant chelate degradation occurred. The sulfur recoveredwas about 99 percent of that which was added as sulfide. The sulfur wasbright yellow, contained 43 parts per million of iron, and was composedof ten micron particles in the form of 100 micron agglomerates.

While this invention has been described with reference to certainspecific embodiments, it will b recognized by those skilled in the artthat many variations are possible without departing from the scope andspirit of the invention, and it will be understood that it is intendedto cover all changes and modifications of the invention disclosed hereinfor the purpose of illustration which do not constitute departures fromthe spirit and scope of the invention.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:
 1. A process for removinghydrogen sulfide from a sour, fluid stream comprising:(A) contactingsaid sour, fluid stream in a contact zone with a first aqueous,alkaline, scrubbing solution at controlled pH and at a temperature belowthe melting point of sulfur, said solution comprising at least onehigher valence state polyvalent metal chelate, formed by reactingpolyvalent metal with an acid or an alkali metal, or ammonium saltthereof where said acid is selected from the group consisting of anamino carboxylic acid, a polyamino carboxylic acid, and aminopolycarboxylic acid, and a polyamino polycarboxylic acid, present insaid solution in an effective amount and suitable for oxidizingsubstantially all the hydrogen sulfide in said fluid stream to produce asweet fluid stream and a second aqueous, alkaline solution comprisingsulfur and at least one lower valence state polyvalent metal chelate:(B) separating in a separation zone said sulfur from said secondaqueous, alkaline solution; (C) passing said second aqueous, alkalinesolution from said separation zone to an anode compartment of anelectrochemical cell, said cell comprising a high surface area porousanode in an anode compartment and a cathode in a cathode compartment,said anode and cathode compartments separated by a permselectivemembrane and said porous anode and said cathode connected through anexternal electrical circuit; (D) regenerating said chelate present insaid second aqueous, alkaline solution in said anode compartment of saidcell to produce said first aqueous, alkaline solution by oxidizing atsaid anode said lower valence state polyvalent metal chelate to a highervalence state; whereby said chelate in said second aqueous, alkalinesolution is oxidized to produce a regenerated aqueous, alkaline solutionwithout substantial degradation of said polyvalent metal chelate bycontrolling the anolyte pH in said electrochemical cell and bymaintaining the anode potential of said cell above the oxidationpotential of the lower valence state polyvalent metal chelate but belowthe oxidation potential of the chelate portion of said polyvalent metalchelate; and (E) recycling said regenerated aqueous, alkaline solutionfrom said anode compartment of said electrochemical cell to said contactzone.
 2. The process of claim 1 wherein said porous anode is a porousgraphite anode, said process is continuous, said second aqueous,alkaline solution is regenerated without degradation of said chelate,and said polyvalent metal chelate is formed by reaction of a polyvalentmetal with an acid, an alkali metal, or an ammonium salt thereof whereinsaid acid is selected from the group consisting of an amino carboxylicacid and an amino polycarboxylic acid.
 3. The process of claim 2 whereinsaid pH in said contact zone is controlled at about pH 6 to about 10 andthe polyvalent metal in said chelate is selected from the groupconsisting of iron, copper, and manganese.
 4. The process of claim 3wherein the anolyte pH of said electrochemical cell is controlled atabout 6.5 to about 7.5, said polyvalent metal is iron, said acid is anamino carboxylic acid, and said anode potential is below about +0.4volts.
 5. A process for converting at least one lower valence statepolyvalent metal chelate to a polyvalent metal chelate in which themetal is at a higher valence state comprising:(A) introducing acontrolled pH, aqueous, alkaline solution of said lower valence statepolyvalent metal chelate into an anode compartment of an electrochemicalcell, said cell having a porous anode having a high surface area and apermselective membrane separating said anode compartment from a cathodecompartment, (B) electrochemically oxidizing said lower valence statepolyvalent metal chelate to a higher valence state polyvalent metalchelate by maintaining the anode potential of said cell above theoxidation potential of the polyvalent metal chelate but below theoxidation potential of the chelate portion of said polyvalent metalchelate.
 6. The process of claim 6 wherein said porous anode is a porousgraphite anode and said anode potential is maintained at about +0.4volts for the conversion of a ferrous chelate of the trisodium salt ofN-(hydroxyethyl) ethylene-diaminetriacetic acid to the ferric valencestate of said chelate.
 7. The process of claim 6 wherein the anolyte pHof said electrochemical cell is controlled at about 6.5 to about 7.5.